One form of screening apparatus which has received wide spread acceptance, particularly in the paper industry, is that which includes a perforated cylinder through which the material is screened and a rotor for maintaining the cylinder perforations open. Screens of this general type are described in U.S. Pat. No. 2,835,173 and 3,849,302. Additionally, U.S. Pat. No. 3,713,541 discloses a screen having a particular slot pattern.
A problem which is characteristic of screens of this type, however, is cracking of the screen cylinder. The perforated screen cylinders, in most applications, are used in environments which require materials of construction which are highly susceptible to fatigue failure. This is compounded by the severe forces imposed on the cylinder by the rotor, and is particularly true where the rotor comprises foils which generate alternate positive and negative pressures as they move along a face of the screen cylinder, resulting in a cyclic, pulsing load being imposed on the cylinder.
To combat the cycle loads generated by the rotor, the cylinder is often provided with circumferentially extending, unperforated bands and, particularly in high speed, high capacity units, reenforcing bands which encircle the screen cylinder and are attached thereto by welding.
One type of failure has been noted, however, which is not obviated by the use of reenforcing rings. This failure, which also involves cracking, is noticed initially adjacent the end of the cylinder where it is attached to the main casing enclosing the cylinder. Despite the frequency with which this type of failure occurs, efforts to determine the cause of and find a solution to these latter failures have been largely unsuccessful, and the conventional response has been merely to accept the fact that failures of this type are inevitable and simply to replace the screen cylinder upon failure.
Additionally, welding of the intermediate reenforcing rings to the screen cylinder to resist radial stresses often creates flaws in the cylinder at undesirable areas near the perforated areas of the screen and undoubtedly contributes to screen failure.